Asbestos fiber-containing articles such as gaskets, brake linings, pneumatic tires, conveyor belts, timing belts, power transmission couplings, shock absorbers, sealants, paints, and the like, are well-known and have achieved significant commercial success. Unfortunately, recent medical evidence indicates that asbestos fibers can cause health hazards, particularly when inhaled. These health hazard problems of asbestos fibers are well-known also, and accordingly, the industry has been searching for replacement compositions for forming gaskets, and the like, which do not contain asbestos fibers, but yet achieve substantially the same physical and chemical properties.
Acrylic fibers have been added to organic thermosetting plastic matrices to improve one or more properties of the matrices. For example, DeMaria and Relmond in U.S. Pat. No. 4,352,852, assigned to the same assignee as the instant application, show that from about 10 to about 90 weight percent of an acrylonitrile polymer fiber having a straight tenacity of at least about 5.0 grams per denier, reinforces thermosetting resins, such as polyesters and peroxide curatives, much better than the widely used glass fibers. The required tenacity is obtained by post stretching a conventional wet spun acrylonitrile polymer fiber in steam under pressure to obtain a higher than normal tenacity. The fibers used are of conventional diameter, i.e., they are "normal denier" fibers, having a diameter of about 20 microns, i.e., greater than about 12 microns.
Greenman, et al, in U.S. Pat. No. 4,423,109, describe fiber reinforced rubber gasket materials, in which one of the fiber materials can comprise glass fibers, and another can comprise polyacrylics, e.g., polyacrylonitrile fibers. No preference is stated or disclosed as between glass, on the one hand, or polyacrylics, on the other, and nothing is suggested that would teach using fibers with diameters in the "microdenier" range, i.e., those less than 12 microns in diameter, and especially from about 5 to about 10 microns in diameter.
Many other proposals have been put forth to replace asbestos fibers in a variety of end uses. Mention can be made, for example of U.K. Patent No. 1,179,569 (tires); EPO Patent Publication No. 0 511 838 (marine couplings and timing belts); Research Disclosure No. RD 276096A (4/10/87) and U.S. Pat. No. 4,820,585 (sealants and coatings); and U.S. Pat. Nos. 4,330,442, 4,387,178, 4,423,109, and 4,866,109 and Japanese Patent Publication No. J03045685 (2/27/91) (gaskets).
Especially important are fiber replacements for asbestos fibers in gasket-forming compositions, because, in order to meet government regulations regarding the emissions of certain gases, such as hydrocarbons and halogens, gaskets in mechanically jointed flanges must be able to maintain their seal and prevent leakage of fluids and gases even when subjected to an extremely severe operating environment and undesirable mechanical conditions. Thus, these gaskets must be able to function even though the flanges are either poorly designed or poorly machined and must maintain the ability to seal the flange and prevent leakages even under high operating temperatures and high internal pressures. Gaskets produced from many prior art materials proposed to replace asbestos fibers, either leak or blow out when exposed to the aforementioned conditions and environments. A significant practical drawback in the prior art gasketing materials is less than optimum flexibility and lower than optimum elongation. This is especially noted with glass or cellulose fibers or polyaramide fibers as the reinforcement, primarily because of their lack of ability to provide adequate elongation and flexibility to the gasket. Flexibility is important from a practical standpoint because a gasket must be able to survive handling, cutting and packaging procedures prior to installation, without cracking or loss of design geometry. Normal denier acrylic fibers provide many advantages in such uses over the others mentioned because they are uniquely adapted to elastomeric matrix resin wet-out, even without the use of sizing agents on their surface to "couple" to the resin and thereby enhance wetting and reinforcement.
Typical of state-of-the art compositions and methods are those disclosed in Hibbard, et al, U.S. Pat. Nos. 4,837,281, and 4,866,109, which describe making reinforced gasketing materials by cutting acrylic fibers and using them to reinforce rubbers which are converted to cured sheets for use as gaskets. These citations do not disclose the use of "microdenier" fibers, however.
Lindeman et al, U.S. Pat. No. 4,330,442, describe asbestos-free gasket forming materials by a beater-additive process in which the fibers comprise phenolic fibers mixed with aromatic polyamide fibers. As will be shown hereinafter these fibers do not provide end products having the optimum elongation and they are not "microdenier" fibers.
Tracey et al, U.S. Pat. No. 4,387,178, describe compressible soft asbestos-free gasket materials by a beater-additive process in which the fibers comprise fibrillated aromatic polyamide fibers. As will be shown hereinafter such fibers do not provide end products with the optimum elongation and the compositions are not made with "microdenier" acrylic fibers.
Nakahara et al, EPO Patent Publication No. 511,838-A1, is also of interest in this connection because it deals with fibrillated polyacrylonitrile short (but not less than 10 microns in length) fibers dispersed in a rubber, to provide high strength and high modulus and good rubber fiber interaction. The compositions are vulcanizable with organic peroxides and also with sulfur to produce timing belts, marine materials, chains, rubber springs, and the like, all uses formerly employing asbestos.
In general, with particular reference to acrylic fibers, e.g., fibers of a co-polymer containing acrylonitrile in a quantity more than 85 wt. %, these have good extensibility, which prevents their fragmentation during mixing, and they adhere well to most resin systems because of the polar nitrile groups present in the molecule. These attributes make them desirable materials to consider for use in replacing asbestos, in uses where flexibility enhancement and extensibility in resin systems is needed while maintaining or increasing the strength of the resulting resin-fiber composite. Modacrylic fibers with more than 35 wt. % and generally less than 85 wt. % acrylonitrile in the co-polymer provide also good extensibility, resistance to fragmentation during mixing and good adhesion to most resin systems, while also providing other attractive attributes such as fire retardancy. Consequently, they also are desirable materials for replacing asbestos.
In summary, the citations above show that, in the present state of the art, fibrous organic reinforcements are known to be suitable replacements for inorganic reinforcing fibers, like glass, in materials for making gaskets, belts, tires, sealants, and the like, and the fibers may specifically comprise acrylic fibers. Microdenier acrylic fibers having diameters of below about the normal 12 microns of the prior art, however, have not been described as reinforcing agents for elastomeric rubbery resin matrices, and especially no acrylic fiber reinforcements for gasket materials having such small diameters have been described for use as reinforcing agents for gasket materials, and the like.
It has now been discovered, and is the subject of this invention, that selecting and using acrylic fibers produced by means of a process which provides very small diameters of not greater than about 12 microns results in an unexpected enhancement in their ability to increase flexibility of elastomeric resin binder systems for materials to produce gaskets, shock absorbers, rubber springs, pneumatic tires, sealants, and the like, and at the same time increases resin-fiber bonding and gasket strength. For example, as will be shown in the accompanying drawing, keeping all other variables constant, reducing the fiber denier from 10 to 0.5, will increase the elongation of a model gasket composition by 20 to 100%. Surprisingly also, this is accomplished with a concomitant increase in tensile strength. Such a reduction would not have been predicted because, generally speaking, if the stiffness of a reinforcing fiber is lowered, the elongation of that fiber and a gasket containing it would be expected to increase. However, since the fiber is then less stiff, reinforcement will be sacrificed and the gasket strength will suffer. Therefore, based on the data presented hereinafter it is assumed that the fiber properties do not change significantly with diameter, so that solely due to small diameter, gasket elongation and strength are both enhanced. Such results are nowhere foreshadowed by the prior art and demonstrate manifest advantages in using fibers prepared in accordance with the present invention as reinforcing agents for materials to make gaskets and the like. It will also be shown that the effectiveness of the microdenier acrylic fibers as a reinforcement is greater than other organic fibers, such as aromatic polyamides, especially in elongation characteristics which are vitally important in gaskets.
It is a principal object of the present invention to provide materials for gaskets and the like, reinforced with microdenier acrylic fibers, i.e., fibers with a denier less than 10, and especially less than 1.0, to enhance flexibility and to increase the strength of gaskets manufactured from such materials, while maintaining satisfactory mixing, processing and surface characteristics. The fibers can be unfibrillated or in the form of a pulp. The comparative examples will demonstrate that the fibers provide excellent reinforcement and flexibility properties because of their extremely small diameter, all other variables being constant. The comparative examples will show that upon curing the acrylic fibers will improve the mechanical properties of the material, by virtue of their extremely small diameters, all other variables being constant. Extensibility (elongation) of elastomeric materials is also increased compared to other fibrous reinforcements, other than asbestos, which has harmful effects on the environment. Because of the good UV resistance and low moisture absorption of the acrylic fibers, they are well suited for outdoor applications. Also, because of their good hydrocarbon and other chemical resistance they are well suited as automotive parts, tires, fuel tank and chemical tank sealant components, as well as in gaskets.